Hypertension is a common disease which can have serious adverse consequences, including an increased risk of stroke, damage to organs including the heart, kidneys, brain, blood vessels and retinas. However, while hypertension is serious and numerous medications exist which attempt to control hypertension, in many cases it remains difficult to manage. For many patients, medications only partially reduce blood pressure and the patients remain at risk.
The difficulty in controlling blood pressure may be due to the complex nature of blood pressure maintenance by the body. Blood pressure is affected by multiple interrelated factors including cardiac activity, the degree of vasoconstriction/vasodilation, the degree of sympathetic stimulation, kidney function, salt and water consumption and balance, the amount of renin/angiotensin produced by the kidneys, and the presence of any abnormalities of the sympathetic nervous system, as well as possibly other unknown factors.
The kidneys play a key role in blood pressure regulation. Sympathetic nerve stimulation to the kidneys results in the production of renin, retention of sodium and water, and changes in renal blood velocity, all of which lead to increased blood pressure. Through a system of interactions with other organs, the production of renin ultimately leads to the production of aldosterone, which causes the conservation of sodium, the secretion of potassium, increased water retention and increased blood pressure. An interruption of the renin-angiotensin-aldosterone system is, therefore, one method of reducing hypertension. For example, therapeutic agents such as angiotensin converting enzyme (ACE) inhibitors, angiotensin receptor blockers (ARBs), and renin inhibitors reduce blood pressure by affecting this system. More recently, attempts have been made to reduce renin production and, therefore, reduce blood pressure by surgically transecting the sympathetic nerves to the kidneys to prevent sympathetic stimulation of the kidneys.
Recent studies have successfully reduced blood pressure in hypertensive patients through the use of ablation of the sympathetic nerves within the renal arteries. The ablation is performed through a catheter and radiofrequency (RF) energy is applied to the interior of the arteries in linear arcs that extend circumferentially around the artery. A single arc may extend around the entire artery or a series of arcs may be created. The arcs in the series of arcs may be spaced apart longitudinally somewhat but are overlapping radially such that the entire inner circumference is ablated by a line of ablation at some point along the length of the artery. In either case, the result is that the ablated arcs transect all nerves running through the walls of the renal arteries. By encircling the arteries with lines of ablation, the surgeon is sure to transect the renal nerves, even though the actual locations of the nerves are unknown.
Because renal artery ablation surgeries have only been performed relatively recently, the long term effectiveness and the risk of long term side effects from such surgeries is unknown. Due to the vital nature of the kidneys and the necessity of maintaining adequate blood velocity to these organs, the risk that such surgeries could lead to scarring and stenosis of the renal arteries is an important consideration. If significant stenosis were to occur, the result could be a loss of kidney function, which could be more problematic than the initial hypertension. A more refined approach to renal nerve ablation is, therefore, desirable.